Gallium nitride, which is a semiconductor material, has become a research focus at present because of its features such as a wide band gap, a high saturation drift velocity of electron, a high breakdown field strength and a good thermal conductivity. In terms of electronic devices, gallium nitride is more preferable for manufacturing devices at high-temperature, high-frequency, high-voltage and high-power as compared with silicon and gallium arsenide. Thus, the electronic device based on gallium nitride has a good prospect.
Since a two-dimensional electron gas is formed in the AlGaN/GaN heterojunction structure, a High Electron Mobility Transistor (HEMT) generally made by AlGaN/GaN heterojunction is naturally a depletion device. It is not straightforward to achieve an enhancement-mode device using AlGaN/GaN hetero-structures. However, the depletion-mode device has its limitation in many applications. For example, an enhancement-mode (normally-off) switching device is needed in the application of the power switching device due to safety requirements. The research of enhancement-mode gallium nitride switching devices has important significance, because it has a great potential in high-frequency, power switching, digital circuit applications and the like.
It is necessary to find a method for reducing the carrier concentration in the channel under the gate electrode in the case that the gate voltage is zero, to achieve the enhancement-mode gallium nitride switching device. A first method is to provide an etching structure under the gate electrode and reduce locally the thickness of an aluminum gallium nitride layer under the gate electrode, to control or minimize the concentration of two-dimensional electron gas under the gate electrode. As shown in FIG. 1, a buffer layer 1, a gallium nitride layer 2 and an aluminum gallium nitride layer 3 are sequentially located on a substrate 10; and a gate electrode 4, a source electrode 5 and a drain electrode 6 are located on the aluminum gallium nitride layer 3. The aluminum gallium nitride layer under the gate electrode 4 is etched locally, and thus the thickness of the aluminum gallium nitride layer in a gate region is reduced. A second method is to form a p-type nitride under the gate electrode, Fermi level in the AlGaN/GaN hetero-structure is pulled up by the p-type nitride (Al) GaN to form a depletion region, and thus the enhancement-mode device is achieved. A portion of a p-type nitride 7 under a gate electrode 4′ is retained selectively, as shown in FIG. 2.
However, these two methods have some disadvantages. In the first method, the threshold voltage is generally in the range of 0V to 1V, which does not reach the applied threshold voltage of 3V to 5V. To achieve a higher threshold voltage, it is needed to provide an additional dielectric layer, such as an aluminium oxide layer formed by atomic layer deposition. But, it is an issue how to control the interface states between the dielectric layer and the AlGaN surface. In the second method, it is necessary to remove the planar p-type GaN over the AlGaN barrier layer. It is difficult to achieve the precise control of the etching depth. Furthermore, defects due to etching and the residual magnesium atom from the p-type aluminum gallium nitride may lead to severe current collapse. In addition, the concentration of the two-dimensional electron gas in the AlGaN/GaN heterojunction is limited since the hole concentration is low (in general, the hole concentration of the p-type gallium nitride is not larger than 1E18/cm3). If the electron concentration of the two-dimensional electron gas is too high to be depleted, the enhancement-mode device can not be achieved. Therefore, the composition of aluminum in the AlGaN/GaN heterojunction of the enhancement-mode device is generally less than 20%, for example, about 15%.